I've just got into the mine after getting the key elsewhere and have reached the second 'checkpoint' at which the symbol appears to indicate the game has been saved. This checkpoint is where you go up a level (machine gun turret) then through an actual cave and into another area through a passage in the rock.

When I got near the 'lamp post' it turned from red to green and the save game symbol appears. I then quit the game and restarted a bit later, only to be back at the beginning of the mine, though with less enemies to fight to get back to the same progress point.

I've noticed that although saves happen at points through the level, you always go back to the beginning of the level but fight through again with less enemies - completed objectives and tasks do remain completed, so it's not as if the savegame data has been lost.

if you exit the game and restart you have to re-fight all the enemies. in fact if you go in the same areas you will have to re-fight the bosses also. not really a bad thing since the game is based on killing stuff for better weapons.